Searching for information about linux-headers upgrades, I found that
1. it's OK to upgrade the headers without upgrading the kernel, because the developers put lots of work to ensure backward compatibility. Old programs only use the old, backward compatible stuff anyway.
2. one probably has to rebuild the packages that depend on linux-headers, in particular glibc. And that's really important so just rebuild glibc with no further thought.

BUT
1. This begs the question: what's the upside of upgrading the headers while using an old kernel? Why are the developers working so hard? If new programs try to use the new stuff, how will this work? Are there compiler directives that will support new headers with almost-as-good functionality from the old kernels?
2. But "equery depends linux-headers" does not report glibc as one of the packages that depend on linux-headers. So is this hint of "always rebuild glibc" outdated?